1. Field of the Invention
This invention relates to polycarbonate resins and more particularly to flame retardant polycarbonate copolymers having improved critical thickness values.
2. Description of the Prior Art
Polycarbonates derives from reactions involving organic dihydroxy compounds and carbonic acid derivatives have found extensive commercial application because of their excellent mechanical and physical properties. These thermoplastic polymers are particularly suited for the manufacture of molded article products requiring impact strength, rigidity, toughness, thermal and dimensional stability as well as excellent electrical properties.
However, one deficiency of polycarbonate when used in molded articles is the low critical thickness values of polycarbonate polymer, which deficiency tends to limit wall thickness to a value below the critical thickness.
It is known that polycarbonate plastics exhibit high notched Izod (ASTM test D-256) impact values. This value, however, is dependent upon the thickness of the test specimen. Typical notched Izod impact values for a 1/8" specimen are about 16 ft.-lbs./in. These high Izod values result because specimens of 1/8" thickness are thinner than the critical thickness of the polymer and therefore upon impact a hinged or ductile break occurs. On the other hand, 1/4" specimens exhibit a clean or brittle break and give notched Izod impact values of only about 2.5 ft.-lbs./in. The 1/4" specimens are said to be above the critical thickness of the polymer. "Critical thickness" has been defined as the thickness at which a discontinuity in Izod impact values occur. In other words, it is the thickness at which a transition from a brittle to a ductile break or vice versa occurs. Thus a standard impact specimen of polycarbonate polymer thicker than the critical thickness exhibits brittle breaks and those thinner than the critical thickness exhibit hinged or ductile breaks. Further, the critical thickness of a polycarbonate based on bisphenol A with a melt flow of 3 to 6 grams/10 minutes at 300.degree. C. (ASTM D1238) has a critical thickness of about 225 mils.
The critical thickness problem is further complicated when the polycarbonate article is to meet a specified requirement for flammability in applications where high temperature and/or exposure to fire may be encountered. Polycarbonate copolymers based on an aromatic diol and a halogenated diol reacted with a carbonic acid derivative are accepted as effective fire retardant polymers. These polymers exhibit generally acceptable physical properties along with complying with flammability requirements. However, the critical thickness of copolymers employing halogenated diols is very low for example about 130-140 mils with a polymer containing 5 to 6% by weight bromine in the form of a halogenated diol.
One approach to solving the critical thickness problem has been to incorporate polyolefin polymers into the polycarbonate which has substantially improved critical thickness (see U.S. Pat. No. 3,437,631). But along with this improvement has come detrimental effects such as colorant dispersion problems because of the diversity of chemical composition of the two component system and also a lack of transparency since the polyolefin and the polycarbonate are incompatible.
Improvement in critical thickness of polycarbonates has been accomplished by incorporating amounts of a sulfur containing aromatic diol into the polymer chains. This has been effective in both flame retardant and non-flame retardant polycarbonates. Further, it has been found that the sulfur in the sulfur containing aromatic diol provides a synergistic effect with the halogen moieties of a second aromatic diol providing a more flame retardant product. Exemplary of the advantages of incorporating sulfur containing aromatic diols, particularly thiodiphenol type diols, into polycarbonates are those teachings in U.S. Pat. No. 4,043,980. Also of interest in this regard is U.S. Pat. No. 3,250,744 which is concerned with polycarbonates based upon in excess of 20 mol % of thiodiphenol (4,4'-dihydroxy-diphenyl sulfide). A disclosure of the incorporation of tetrabromothiodiphenol (3,3',5,5'-tetrabromo-4,4'-dihydroxydiphenyl sulfide) into low molecular weight polycarbonate oligomers appears in Japanese Published Patent Application (Kokai) 67850/75.
In accordance with the present invention a copolymer is provided which has flame retardant characteristics and improved critical thickness values while remaining highly transparent.